Re: How can it possibly be worth that much?
"Does it sling ads at people?"
Probably it will - not hard to just use an ad blocker.
"Charge money for businesses to keep their code there?"
Of course if they want private repositories and any level of support. Just like now.
I'm sure they will (subject to GDPR optin of course) try to sell you stuff such as developer tools and related services too.